The objectives of this proposal are to elucidate the mechanism by which Chlamydia trachomatis evades the immune response by attenuating IFN?-dependent signaling in infected host cells, and to develop a new reverse genetic strategy to study C. trachomatis pathogenesis and biology. Sexually transmitted C. trachomatis infections, primarily caused by serovars D-F, are a major public health concern with adverse effects on female fertility and neonatal health. Despite significant public health interventions including education, screening and treatment, reported cases n the US reached 1 million in 2006 and continue to rise, resulting in annual health care costs >$2.4 billion. Asymptomatic infections in >70% of women and >30% of men, can take months to years to resolve, and thereby compounding transmission of C. trachomatis. Transmission is exacerbated by short-lived protection to re-infection. In vitro studies in human epithelial cells, and animal studies using C. muridarum, have demarcated IFN? as a major anti-chlamydial mediator. In vivo, although elevated IFN??is found in the infected human endocervix, clinical data indicate that C. trachomatis can evade immune responses. We have developed an innovative approach using a novel penetrant peptide (CPP) to label C. trachomatis Elementary Bodies (EB). Our approach has permitted the isolation of pure populations of infected endocervical epithelial cells, from mixed pools of cells exposed to C. trachomatis. Analyses of pure populations has revealed that C. trachomatis attenuates IFN?-dependent expression of indoleamine-2,3-dioxygenase (IDO1). Attenuation permits C. trachomatis, a tryptophan auxotroph, to ameliorate the effect of intracellular tryptophan depletion by IDO1. The first goal of this application is to understand how Chlamydia blocks host-cell IFN?-dependent signaling. Understanding this mechanism will permit development of new strategies that target immune evasion by C. trachomatis. The paucity of easily applicable genetic tools to manipulate C. trachomatis has limited the capacity to investigate mechanisms employed by C. trachomatis that drive immune evasion and pathogenesis. This application addresses this significant deficiency by proposing an innovative approach using the same CPP to carry cargoes that repress expression of specific C. trachomatis genes. When developed and validated, our strategy will permit robust reverse genetic approaches to molecularly dissect virulence determinants of C. trachomatis. These studies with our novel CPP will significantly shift current paradigms and revolutionize approaches in Chlamydia research by: 1) Establishing a robust method to elucidate previously unidentified immune evasion strategies used by C. trachomatis; and 2) Providing, for the first time, a robust reverse genetic technique to dissect the role of specific bacterial genes in the developmental cycle and pathogenesis.